Light pulse generators using electroabsorbent modulators are applicable in technical fields that require light pulses of short-duration to be generated.
For example, they are used in measurement apparatuses for observing high-speed chemical or physical phenomena.
They are also used in the field of long distance telecommunications for transmission over optical fibers of high bit rate binary data encoded by using soliton pulses or by using conventional RZ modulation.
For a discussion of the present state of the art concerning electroabsorbent modulators, reference may advantageously be made to the article by E. Bigan "Modulateurs electroabsorbants en onde guidee pour liaisons optiques a 1,55 .mu.m" [Guided wave electroabsorbent modulators for optical links at 1.55 .mu.m] published in L'echo des Recherches, No. 149, 3rd quarter 1992.
An electroabsorbent modulator of the type presently known is shown diagrammatically in FIG. 1. Such a modulator is a diode including two thin layers 1 and 2 of material doped to be N-type and P-type respectively. Between these two layers 1 and 2 there is interposed a layer 3 which constitutes a region that is not intentionally doped and in which an electroabsorbent material is included. Such an N-I-P diode is formed to constitute a waveguide structure.
Absorption by the electroabsorbent material in the intermediate layer 3 varies as a function of the voltage V applied across the two thin layers 1 and 2 of the diode.
It is common practice to characterize the transmission of a layer that includes an electroabsorbent material by the ratio of the outlet over inlet powers of a light beam applied to said layer. FIG. 2a of the accompanying drawings shows the transmission curve of the FIG. 1 modulator as a function of the voltage V applied across its two doped thin layers 1 and 2. As shown by that curve, when the diode that constitutes the modulator is forward biased (V greater than zero), current passes through the diode and the electroabsorbent material operates under laser or optical amplifier conditions (part I of the curve).
In contrast, when the diode is reverse biased (V less than zero) it ceases to conduct and an electric field is generated across the electroabsorbent material. The absorption of the material varies as a function of the applied voltage V. In particular, the electroabsorbent material is transparent so long as said voltage is low or zero (part II of the curve) and it is absorbent for voltages of larger absolute value (part III of the curve).
To generate short light pulses, proposals have already been made to control such an electroabsorbent modulator by means of a sinewave voltage signal.
In this respect, reference may advantageously be made to the article "Transform-limited 14 ps optical pulse generation with 15 GHz repetition rate by InGaAsP electroabsorption modulator", by M. Suzuki, H. Tanaka, K. Utaka, N. Edagaw, and Y. Matsuchima, Electronics Letters, May 21, 1992, Vol. 28, No. 11.
As described in that article, such a signal reverse biases the modulator-constituting diode. It is fixed in frequency and its amplitude is much greater than the control voltage of the modulator.
FIG. 2b shows a sinewave signal S for applying voltage control to an electroabsorbent modulator having the transmission curve of FIG. 2a. As can be seen in FIG. 2b, it is only when the sinewave is at peak values close to zero voltage that the signal S takes on values in which it causes the electroabsorbent material to take up the transparent state.
The pulses generated in this way are short pulses at a repetition rate that is equal to the frequency of the control signal. They are shown in FIG. 3.
Such a light pulse generator presents several drawbacks.
Given that the transparent state of the electroabsorbent modulator corresponds to the peaks of the sinewave of the controlling signal, there is limited scope for adjusting the width of the resulting pulses. In particular, the ratio of pulse width over repetition period is greater than some minimum value. The existence of that minimum value prevents presently known electroabsorbent modulators being used for generating pulses that are very short, and in particular prevents such electroabsorbent modulators being used in techniques for time multiplexing a plurality of pulse trains.
Furthermore, with such a light pulse generator, it is necessary to monitor the control voltage signal continuously for possible drift. To avoid jeopardizing the reliability of the device, the control voltage must not exceed the avalanche voltage of the diode. Also, to prevent the electroabsorbent material operating under amplifying or lasing conditions, the control signal must remain negative.